1. Technical Field of the Invention
The present invention relates to a laser annealing method and apparatus for performing an annealing treatment on a workpiece to be processed by irradiating the workpiece with laser light.
2. Description of the Related Art
Hitherto, in order to realize a high-performance device such as a liquid crystal display, a method of forming a high-performance semiconductor device such as a thin film transistor on an insulating substrate such as a glass substrate has been developed. In this semiconductor device, thin-film polycrystalline silicon obtained by crystallizing amorphous silicon by a heat treatment is generally used.
As a method of manufacturing a polycrystalline silicon film, a method of forming an amorphous silicon film in advance on a substrate and crystallizing the film by irradiation of laser light is generally used. According to this method, since it utilizes a crystallization phenomenon in the process of melting and solidification of the semiconductor thin film, it is possible to obtain a high-quality polycrystalline silicon film having a relatively large particle size.
As the laser annealing method that is most commonly used at present, a method that uses pulsed laser light, particularly, an excimer laser is known. In recent years, methods that use a YAG laser or a YLF laser which is a solid-state laser have been developed. The method that uses pulsed laser light involves locally melting an amorphous silicon film and heating the melted area to a high temperature for only a very short period. For this reason, since the high temperature is not transmitted to the substrate, it is possible to use a low-cost glass substrate rather than using an expensive quartz substrate having excellent heat resistance. Thus, such a method is most suitable for low-temperature processes of a large-area electronic device.
In a general laser annealing method, when an excimer laser is used, for example, laser light is shaped by an optical system into a linear beam of which the long-side beam length is 465 mm and the short-side beam length is 0.4 to 5 mm. In the case of a YAG laser, it is possible to obtain a linear beam of which the long-side beam length is 200 mm and the short-side beam length is 40 to 50 μm. The laser light and the substrate are moved relative to each other so that the irradiation position of the linear beam partially overlaps with the irradiation area in the short-side direction of the beam, whereby crystallization of the semiconductor film is realized. In general, the overlap ratio is set to about 95%, and the same area is irradiated with about 20 shots of laser beams.
FIGS. 1A and 1B are diagrams illustrating a conventional laser annealing method. FIG. 1A shows a state in which a substrate 30 is irradiated with laser light 31 while moving the laser light 31 in the short-side direction of the beam relative to a substrate 30. When the substrate 30 of which the size is longer than the beam length is processed, first, a first row R1 is irradiated as shown in
FIG. 1A. When irradiation of the first row R1 is completed, the laser light 31 is relatively moved in the long-axis direction so as to perform irradiation of second and subsequent rows. FIG. 1B shows an example in which the entire surface of the substrate is processed by irradiation of two rows. At that time, a seam portion W of the beam is formed between adjacent rows (in FIG. 1B, the first row R1 and the second row R2).
In general laser annealing, the edges of the linear beams of adjacent rows are irradiated in an overlapping manner, whereby the seam portion W is formed.
In this seam portion W, the overlap (overlapped irradiation of beams) is excessive, and a crystallization quality thereof is different from that of the other areas, whereby the seam portion W is visible to the naked eye. On the other hand, when there is a small or no overlap of the beams of adjacent rows, the overlap becomes insufficient at the edges of the beams. Thus, an amorphous portion remains, and as was expected, the crystallinity thereof is different from that of the other areas.
Since the beam seam portion W is an area which is irradiated with both the beams of the adjacent rows (for example, the first row R1 and the second row R2), the crystallinity of that area is different from that of the other areas. The crystallinity fluctuates greatly in the case of the excimer laser, and although the fluctuation becomes small with a second harmonic wave of a YAG laser compared to the excimer laser, the crystallinity fluctuates as was expected.
When a thin film transistor is manufactured using an area in which the crystallinity is not uniform, it is not possible to obtain a uniform performance of the thin film transistor. In a display apparatus such as a liquid crystal display or an organic EL display, the display quality thereof is likely to depend on the uniformity of the performance of the thin film transistor. Therefore, in order to maintain the display quality, a display panel has to be manufactured within an area that is limited to the beam length of laser light. In the case of a green YAG laser, since the beam length thereof is short, it is difficult to manufacture a large-size display whose size exceeds 20 inches.
Techniques for preventing the non-uniformity of the crystallinity in the beam seam portion W as described above are disclosed in Patent Documents 1 and 2, for example.
The method of Patent Document 1 is to make the crystallinities uniform between the overlapping portion and the non-overlapping portion of the laser light by regulating the width dimension and the energy density of laser light when the irradiation areas of laser light are made to overlap each other so as to crystallize a large-area semiconductor thin film.
The method of Patent Document 2 is to make the crystallinity uniform by setting the beam profiles in a seam portion so that the slope portions of the beam profiles overlap each other in an area where the laser intensity is 20% or higher and 80% or lower.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2000-315652
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2005-243747
However, the method of Patent Document 1 has a problem in that it is not easy to regulate the width dimension and the energy density of the laser light, and productivity of the device decreases.
In the method of Patent Document 2, since the beams overlap each other under the conditions of different energy densities, it is expected that the crystallinity becomes nonuniform in the case of the excimer laser with which the crystallinity fluctuates particularly greatly with variations in energy.